With current KrF photolithography, it is difficult to achieve patterns in photoresist in which the critical dimension is below about 125 nm while retaining adequate manufacturing process latitude. In particular, the manufacture of magneto-resistive (MR) heads for high-density storage applications calls for ever decreasing dimensions of the reader/writer sensors which, in many cases, extend beyond the capability of current lithographic equipment, necessitating the incorporation of resolution-enhancement techniques such as phase shifting masks (PSM), optical proximity correction (OPC), and off-axis illumination.
These techniques, however, greatly increase process complexity and manufacturing cost in exchange for marginal reductions in the manufacturable feature size. Current lithographic equipment is capable of achieving a minimum feature-size of 180 nm while maintaining manufacturable process latitude. With the help of OPC and off-axis illumination the minimum size may reducible to 120 nm. Further reductions below 120 nm will require immense investment in PSM and/or next-generation lithographic tools.
Eisele et al. (U.S. Pat. No. 6,117,622) teach a technique that utilizes post-development UV irradiation and bake of chemically amplified photoresist patterns to shrink the resist features by up to 25%. Their technique, however, has limited practical value because
1. it shrinks the resist unevenly in a direction perpendicular to the substrate; i.e., the top of the resist shrinks to a much greater extent than the bottom, where the adhesion between the resist and substrate prevents effective shrinking;
2. it calls for a high-temperature bake, up to 230° C., after irradiation to flow the resist to achieve large shrinkage, which reduces the uniformity of resist dimensions and is often deleterious to the overall process; and
3. upon UV irradiation and bake, photoresists tend to become insoluble in stripping solutions, requiring additional aggressive resist-removal processes that greatly limit the applicability of their technique.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,348,301, Lin describes linear reduction of a PR mask using two baking steps. In U.S. Pat. No. 8,340,556, Wong discloses electron beam radiation to reduce line width of a pattern and, in U.S. Pat. No. 6,576,405, Buffat et al. teach UV exposure and post-exposure baking.